What causes it, besides getting cyberware or deadly injuries?

Drugs.

Anything that causes Essense loss. Just check page 61 in your SR4 book emo for the larger list. SR4 has, at least for now, removed Magic loss causes outside of Essense loss.

Essence loss, bio index increase (IIRC they made that an errata), drugs, Deadly wounds, and those magical duels they described in Magic in the Shadows. There's a bunch of other stuff that can do it, but it's all temporary.

~J

And remember kiddies burnouts are an excellent chance to introduce the Aleph society =P.

I ran one of my players through a small run with them, and since i had not read up on them in a while i was pleasantly surprised to see that they have a tiny connection to the atlantean foundation, and so does that Player. huzzah for GM luck.

Actually the only real way to Burnout is to lose essece with any of the above plus some more ways. Then take a Geas, then break that Geas.
At this drastic point you can no longer increase your magic through intiation and therefore are truely on the path of the burnout.
All the others can be countered by initiating to a higher level to recoupe lossed magical potential bar a single magic loss to zero.
Some what Twisted,
I think you will agree that takeing a geas to off set magic loss is in part the only way to truely BurnOut.

I do not agree. As you yourself point out, if you simply do not get around to initiating (for whatever reason) before that last point of Magic goes, you have burned out.

~J

How strict do geasa have to be?

Absolutely strict. Either they are upheld or they are broken. No middle ground.

SR3: See MitS. SR4 doesn´t have them so far (I think).

Which is because you can buy your magic att. now. No more burnouts in Sr4.

Getting molested by a vampire too much also does it.